The production of potassium sulfate from potassium chloride and sulfuric acid has been extensively evaluated in the past due to its rarity and commercial value. A Mannheim furnace, which utilizes high temperatures ranging from 450-600° C. to promote the dry conversion of KCl to K2SO4, is conventionally used. Among the drawbacks to this process, one stands out. This process requires large amount of energy, so much energy that the feasibility of the process in some cases in questionable.
U.S. Pat. No. 3,998,935 describes a method in which KCl is reacted with a hot solution of KHSO4 to produce K2SO4 by cooling crystallization. However, because direct contact occurs in a separate reactor followed by the cooling crystallization step, a three stage reaction is required for the method to be completed. Further, this method produces an excess of KHSO4 at some input concentrations.
The method described in U.S. Pat. No. 4,045,543 attempts to reduce the complexity of the method described in U.S. Pat. No. 3,998,935 by eliminating the crystallization of an intermediate salt. This is accomplished by a four stage process: reaction, evaporation, crystallization, and filtration. The primary drawback here, however, is that the process is not economically favorable due to the required step of HCl evaporation which produces a relatively low concentrated solution.
U.S. Pat. No. 4,436,710 suggests a method in which a moderately high temperature (290-350° C.) is used to form a mixed salt of K2SO4 and KHSO4. This is followed by a dissolution step in aqueous phase and cooling crystallization to produce K2SO4, and is in essence a Mannheim-like process step followed by two crystallization steps. As such, it does not substantially improve on energy requirements.
The process described in U.S. Pat. No. 4,588,573 uses the approach of U.S. Pat. Nos. 3,998,935 and 4,045,543 by reacting KCl and H2SO4 while evolving an HCl/H2O distillate. The solution is cooled for crystallization of highly acidic mixed salts, which are converted stepwise from KH3(SO4)2 to KHSO4 to K2SO4. However, this is a multistage process and utilizes an arrangement similar to that described in U.S. Pat. No. 3,998,935.
Finally, WO 03/074424 presents an approach wherein a double salt, K3HSO4, is separated and converted to K2SO4 by dilution into water.
All of the above processes are multi-stage and fairly complex processes, often requiring the production of intermediate salts, or are energy intensive and therefore not economically feasible. Further, the use of cooling crystallization often results in significant KCl crystallization, which is an undesirable impurity in the K2SO4 salts. Therefore, there is a need for a relatively simple process for producing K2SO4 that, compared to many conventional processes, consumes less energy, and does not include multiple stage complex processes and processes that require cooling crystallization.